It is well known that two-terminal semiconductor diodes can show useful response either when energised by passage of current through the terminals-to produce emission of light (operation as a light-emitting diode, LED), or as photovoltaic (PV) or photoconductive (PC) devices where the current through or voltage across the terminals is varied by the level of light illumination falling on the diode. Among the inorganic semiconductors, devices made with III-V semiconductors have been shown to exhibit both these effects in the 'same device. Among thin-film devices made with molecular semiconductors, both LED and PV or PC behaviour has been observed see for example the LED operation reported by Tang and Van Slyde ("Organic Electroluminescent Diodes", C. W. Tang and S. A. VanSlyde, Appl. Phys. Lett. 51, 913-915 (1987)) and PV operation also report by Tang ("Two-Layer Organic Photovoltaic Cell", C. W. Tang, Appl. Phys. Lett. 48, 183-185 (1986).
Thin-film diodes made with conjugated polymers as the active semiconductor layer have been shown to function as LEDs ("Electroluminescent Devices and Their Manufacture", R. H. Friend, J. H. Burroughes and D.D.C. Bradley, U.S. Pat. No. 5,247,190), and it has also been shown that these same devices can function as PV or PC devices ("Electrical and Optical Characterisation of Poly-phenylene-vinylene Light Emitting Diodes", S. Karg, W. Riess, V. Dyakahoy and M. Schwoerer, Synthetic Metals 54, 427-433 (1993)).
Photodetectors have a large number of applications and are frequently used in an array in which each photodetector can sense radiation incident on it and generate an output signal indicative of the radiation. Such arrays are useful, for example, as image sensors.
Until recently, arrays of photo detectors were constructed by manufacturing a plurality of discrete photodiodes on a substrate to form an array. Recent developments have led to the manufacture of thin film amorphous silicon photodetector arrays for use as large area image sensors. Reference is made to a paper entitled "Amorphous Silicon Image Sensor Arrays", Mat. Res. Soc. Symp. Proc. Vol. 258, 1992, Materials Research Society, for details of an amorphous silicon image sensor.
Such arrays, that use thin film technology on glass substrates, can be used in a wide variety of applications. These include sensors, particularly contact image sensors which can read a document placed directly on top of an array.
The present invention provides an alternative material for use in a photodetector device.
Thin-film devices made with conjugated polymers are of particular interest in that they offer the possibility of fabrication of large-area devices with excellent mechanical and structural properties. Unlike thin-film silicon, which is widely used as a PV diode material, conjugated polymers such as poly(phenylene vinylene) and other such polymers as disclosed in "Electroluminescent Device and Their Manufacture", R. H. Friend, J. H. Burroughes and D.D.C. Bradley, U.S. Pat. No. 5,247,190, can function also as LED devices, and there is particular interest in fabricating structures which simultaneously show useful performance as both LED and PV or PC devices. However, until now, although the LED performance of such devices is sufficiently good to be useful for applications as displays, in respect of efficiency of light emission and colour of emission, the PV performance has been less satisfactory, with quantum efficiencies for PV operation typically 1% and energy efficiencies Of considerably less than 1%. These figures are very much lower than that now achieved for thin-film PV devices based on inorganic semiconductors such as amorphous silicon. There are several reasons for the low efficiencies of which the difficulty in achieving efficient ionisation of the excitons initially created by photon absorption is probably the most important, though poor electron or hole mobilities are also problematic. Ionisation of excitons which have binding energies significantly above kT, where k is Boltzmanns constant and T is the temperature in .degree. K.,k can be achieved in the bulk but is often facilitated by the presence of a surface or interface. At a heterojunction between molecular semiconductors with differing electronegativities, ionisation to place electrons in one layer and holes in the other can be arranged to be energetically favoured, and some of the best photovoltaic responses have been obtained from devices of this type.
Conjugated polymers have been tried in photocells at an early stage, but did not produce very promising results, with relatively low open-circuit voltages and low overall efficiencies. Initial efforts were with polyacetylene and some of the polythlophenes. The low open-circuit voltages were attributed in part to the effects of polaron formation, which appear to move one-electron levels deep into the semiconductor gap, so that the energy difference between the energies of the one-electron hole and electron levels can be very much smaller than the .pi.--.pi.* energy gap. The extreme case is that of trans-polyacetylene, where the excitations take the form of solitons with non-bonding levels at mid-gap, and which can be either positively or negatively charged, so that the one-electron levels for electrons and holes are coincident. The chemical potential for these carriers must also take the lattice polarisation into account, so that larger photovoltages can be produced.